In recent years, office machine such as copying machines, laser printers and the like aim at high image quality, high-speed printing, and colorization, and in order to realize them at the same time, they tend to be of a so-called tandem engine type each including a photo sensitive drum for each color. Increase in size of apparatuses is inevitable since the apparatuses, which conventionally have the constructions of one drum, adopt the constructions of two drums and four drums, but in order to suppress an increase in size, the main motors which drive various kinds of mechanisms are required to be thin, small in diameter, space-saving and high in output power.
As the main motor, a brushless motor, in which a motor part including a stator with a stator core (iron core) suitable for high output power and a drive/control circuit for rotationally driving and controlling the motor are integrally mounted, is generally used, and is required to be thin, small in diameter, space-saving and high in output power as the whole motor including the circuit part.
For this purpose, it is desired that the outside diameter dimension of the rotor part of the motor is made small, the outside dimension of the circuit part is made as close as possible to the dimension of the square which circumscribes the rotor part, and the height dimension is also made as close as possible to the height dimension of the rotor part.
Conventionally, a brushless motor in which a circuit part and a motor are integrally incorporated generally has the construction including a stator with a coil wound around a stator core, a rotor in which a ring-shaped magnet having a plurality of magnetic poles opposed to the stator core is fixed to an inner peripheral surface of a cup-shaped rotor frame and a shaft is mounted at a center, bearings which support the shaft of the rotor, and a housing which holds the bearings, in which the stator core and a circuit board on which circuit components are mounted are attached to the housing (see Japanese Utility Model Laid-open No. 7-9067, for example).
FIG. 14 shows a structure of the first prior art example.
Reference numeral 18 denotes a rotor frame (cup part), reference numeral 20 denotes a rotor magnet (driving magnet), which is fixed to the rotor frame 18 with a speed detecting magnet 22, and a shaft 24 is mounted to a center of the rotor frame 18, whereby a rotor 16 is constructed. The shaft 24 is rotatably supported by bearings 28 and 28 held by a housing 12.
A stator core 48 which is disposed to be opposed to the rotor magnet 20 and a circuit board 30 are attached to the housing 12, and a drive/control circuit having a controlling integrated circuit 34 for rotationally driving and controlling the rotor 16 is mounted on the circuit board 30.
However, in this conventional construction, a space for inserting a bolt 46, which fixes the stator core 48 and the circuit board 30 to the housing 12, through is required in an inner peripheral part of the stator core 48. Further, a space for fitting an outer peripheral portion of the housing 12 which holds the bearings 28 and 28 in its inner periphery into a hole formed in a center of the stator core 48 is also required.
In order to decrease an outside diameter of the rotor 16, an outside diameter of the stator core 48 also needs to be decreased, and therefore, there arises the problem that the space for winding a coil 14 around the stator core 48 decreases and the output power of the motor significantly reduces.
As an example of fastening with a bolt as in FIG. 14, an example shown in FIGS. 15 and 16 is conceivable. In this example, a base end portion of a housing 2 is fixed to a plate 1, the circuit board 30 on which the drive/control circuit is mounted is screwed onto the housing 2 with bolts 31, a stator core 33 on which a winding wire 32 is wound is fixed to the housing 2 with bolts 34 as shown in FIG. 16. In this case, there also exists the same problem as the first prior art example.
In order to solve the problem, there exists Japanese Patent Laid-open No. 11-89196 shown in FIGS. 17 and 18.
In the second prior art example, at fitting portions of the housing 2 which holds bearings 11 and 12 for supporting a shaft 6, a stator core 3 on which a winding wire 5 is wound, and a plate 1, a projected part 2a is formed on an outer peripheral portion of the housing 2 and a recessed part 3a is formed on an inner peripheral portion of the stator core 3, respectively. After the projected part 2a of the housing 2 and the recessed part 3a of the stator core 3 are engaged with each other, they are fixed by crimping.
As described above, according to the second prior art example, a space for a fastening portion of the stator core 3 and the housing 2 can be made smaller than the case of the first prior art example, and therefore, a space for winding more coil around the stator core 3 can be secured. Therefore, a highly efficient motor is obtained.
However, in the second prior art example, the problem of requiring a space for fitting the housing 2 in the inner peripheral portion of the stator core 3 is not solved.
The present invention solves the above conventional problem, and has an object to provide a brushless motor in which a drive/control circuit is incorporated integrally with the motor and which is designed to be thin, small in diameter, space-saving and high in output power.